1. Field of Invention
The present invention relates to a method for designing a mask pattern. More particularly, the present invention relates to a method for designing an assistant pattern surrounding a main pattern on a mask.
2. Description of Related Art
As the integrated circuit is highly developed, the reduction and integration of the device become an inevitable tendency and are the main development task for the industry. In the manufacturing process of the integrated circuit, the photolithography process becomes a primary key for determining the device properties. With the increasing of the integration of the device, the size of the device is reduced and the distance between the devices is reduced as well. Therefore, the deviation occurs as the pattern is transferred from the mask to the material layer in the photolithography process. The deviation is relatively small when the size of the device is large and the integration of the device is low. Nevertheless, in the integrated circuit with high integration, the device properties are seriously affected. Therefore, the electrical performance of the device is seriously affected.
In order to overcome the pattern distortion problem occurring while the pattern is transferred, several assistant patterns are interleaved between the device patterns on the mask so that the distortion of the device pattern occurring during the device pattern is transferred can be reduced by utilizing the optical effect. FIG. 11 is a top view schematically showing a conventional mask pattern. As shown in FIG. 11, a main pattern 12 on a semiconductor layout 10 to be transferred is provided. For each edge of the main pattern 12, there is an assistant pattern 14 arranged apart from the corresponding edge of the main pattern 12. During designing the arrangement of the assistant patterns 14 on the layout 10, each assistant pattern 14 is arranged along the corresponding edge of the main pattern 12 and then each assistant pattern 14 is shortened to prevent from overlapping each other. More particularly, around the corners of the main patterns, the coverage ratio of the assistant patterns to the main pattern is worse. Hence, the total coverage ratio of the assistant patterns 14 to the main pattern 12 is poor. Therefore, the lithography weak points occurs around the poor coverage region of the main pattern 12 and the distortion of the resulted pattern on the target material layer after the photolithography process happens as well.